1. Field of the Invention
The present invention relates to a method of fabricating semiconductor devices such as thin-film transistors (TFTs) and, more particularly, to a method of fabricating semiconductor devices using a silicon film which is crystallized by the catalytic action of a metal element.
2. Description of the Related Art
In recent years, a configuration using TFTs as liquid crystal devices has attracted attention. This is known as an active matrix liquid crystal display and has millions of pixels arranged in rows and columns. TFTs are connected with each pixel. This liquid crystal display is characterized in that the manner in which electric charge is stored in pixels and transmitted out of them is controlled by these TFTs. This active matrix liquid crystal display is capable of providing a display with high information content and at a high speed. Consequently, the active matrix liquid crystal display is used in portable workprocessors and computer displays.
Although amorphous silicon film is conveniently used as a silicon film incorporated in TFTs, the electrical characteristics of the amorphous silicon film are much poorer than those of single-crystal semiconductor used in a semiconductor integrated circuit. Therefore, TFTs using the amorphous silicon film can be used only in limited applications such as switching devices in active matrix circuits.
In order to improve the characteristics of a TFT, a silicon film having crystallinity may be used. Besides single-crystal silicon, polycrystalline silicon film and crystallite silicon film are known as silicon films having crystallinity. In order to obtain a silicon film having such crystallinity, an amorphous silicon film is grown and then heated by thermal annealing to crystallize the amorphous film. This method is known as solid phase epitaxy, because the crystal state is changed from amorphous state to crystalline state while the solid phase is maintained.
Generally, liquid crystal displays are required to use substrates having transparency and so limitations are imposed on the substrate material. Generally, a material which satisfies the various requirements, i.e., it has transparency, is cheap, and provides a large area, is only glass.
Where silicon is grown by solid phase epitaxy, the heating temperature is above 600° C. and the heating time is more than 10 hours. Corning 7059 glass which is widely accepted into general use has a strain point of 593° C. Where increases in area of substrates are taken into account, it is difficult to perform thermal annealing above 600° C.